1. Field of the Invention
The present invention relates to a package fabrication method using laser illumination. More particularly, the present invention relates to a Wafer Level Package (WLP) fabrication method capable of welding a lid wafer with a device wafer by using laser illumination.
2. Description of the Related Art
As well known in the art, electronic devices such as a Film Bulk Acoustic Resonator (FBAR) and a Surface Acoustic Wave (SAW) filter are being rapidly reduced in size as well as required of high reliability.
In these days, electronic devices are being rapidly reduced in size and thickness according to various requirements such as functional improvement and reduction in power consumption and device size. FBARs largely used in mobile phones are also following this trend. In the FBARs, accordingly, Chip Size Packages (CSPs) are gradually drawing attention, and Wafer Level Packages (WLP) are also being attempted in various aspects in order to improve productivity while reducing fabrication cost. Such a WLP is generally regarded as a major decisive factor in the competitiveness of a product in the market.
A WLP FBAR as above includes a device wafer having a micro drive unit such as a circuit mounted thereon, a lid or cap wafer coupled to the top of the device wafer and a side wall made of a metal strip for coupling the periphery of the device wafer with that of the lid wafer while sealing an air cavity or internal space designed for seating the drive unit from the outside. The cavity functions to protect internal electrode patterns from bad effects of hostile environments and foreign materials.
WLP electronic devices are fabricated according to a conventional fabrication process as follows: First, a drive unit is formed on a wafer, a cavity surrounding the drive unit is formed, and then a cap or lid is covered on the cavity via adhesive or other suitable bonding techniques to seal the cavity. Since any materials deposited on the top of the drive unit degrades the performance of a device, it is important to separate the lid from the drive unit to a predetermined gap.
Generally in the conventional fabrication process, a plurality of drive units and cavities are generally formed on a bulk wafer, and the bulk wafer is cut into a plurality of package-sized unit wafers each having a drive unit and a cavity before the lid is covered on each of the unit wafers via adhesive and so on.
However, this process has a problem in that following process steps are difficult since the bulk wafer is cut into small package-sized unit areas. In addition, since the lid is coupled to the wafer via adhesive and so on, there is a drawback in that a junction is fragile to external thermal impact.
As an improvement, such a wafer level electronic device can be fabricated into a WLP based upon a technique for designing RF device packages, particularly, eutectic metal bonding.
The eutectic metal bonding hermetically couples a cap or lid to a wafer by using a low temperature thermal reflow technique or an ultraviolet lamp technique. In more detail, a bonding metal strip is formed around a drive unit on a device wafer thereby forming a cavity, a lid is covered on the strip, and the bonding metal strip is melted via the low temperature thermal reflow technique or the ultraviolet technique to couple the lid to the device wafer. Alternatively, a corresponding metal strip may be formed on the lid to bond with the metal strip on the device wafer.
However, the above low temperature or ultraviolet lamp technique has following drawbacks. First, since these bonding techniques are performed at a relatively low temperature, the bonding metal strip is heated for a long time exposed to a heat source and thus the drive unit of the device wafer is also heated along with the bonding metal. This as a result exposes the drive unit to the heat source for a long time, excessively raising the temperature of the drive unit, so that thermal impact is applied to the drive unit degrading its properties. In particular, these bonding techniques need a relatively long working time (e.g., several minutes), and thus cause a higher risk of degradation to the drive unit. In addition, the long bonding operation of the prior art increases the entire process time and thereby lowers the productivity of a packaging process.
In order to overcome the foregoing drawbacks, it is required for the bonding metal strip to couple the upper lid wafer and the lower device wafer together for a short time period in a relative low temperature range which does not give thermal impact or effect to the device drive unit. However, since the device drive unit is made of heat sensitive material such as Au, those metals can rapidly weld via the thermal reflow at a temperature range, which is low enough not to give any thermal impact to the heat sensitive material, are very rare.